Irritating sounds are oftentimes problematic in a wide range of settings including, for example, offices, homes, libraries, cars, outdoor roadways, construction sites, and industrial locations.
Broadly speaking, there are two types of noise reduction, which can be used alone or in combination. The first is passive noise reduction, which is generally achieved by insulating the ear from the external noise. Headphones may be insulated with material that prevents noise from reaching the ear. A room may use techniques known in the art as soundproofing to reduce an occupant's perception of noise coming from outside the room.
The second type of noise reduction is active noise reduction (“ANR”), being a method for reducing noise by emitting a second sound that cancels the unwanted noise. Known algorithms are able to analyze the waveform of a noise, and generate a signal that shifts the phase, or inverts the polarity of, the noise. When a first sound wave meets an inverted (also referred to as “antiphase”) sound wave that is equal in both frequency and amplitude, the first and second sound waves effectively cancel each other out. Similarly, when a first sound wave meets a second sound wave with either more or less frequency and amplitude, the first wave is either reduced or amplified accordingly. Generally, active noise reduction as it is currently employed is effective in small areas such as the user's ears for headphones and for hearing aids, and ineffective at larger disperse areas. Using ANR in hearing aids is in addition to their use for amplifying frequencies for hearing impaired. Using ANR in headphones is in addition to their use for playing music.